In the past, small tractors have included circuits that provide an indicator to the operator, such as a solid light on the vehicle dash, if the operator sets the park brake. However, these circuits may turn on the solid indicator light even if the operator sets a park brake that is adjusted incorrectly and cannot hold the tractor in place, or if the operator sets the park brake while the tractor is moving.
Various interlock devices have been proposed to keep a tractor or other work vehicle from moving while the park brake is set. For example, U.S. Pat. No. 3,912,056 relates to a mechanical interlock activated by the park brake to restrict the operator's ability to apply the throttle pedal. Fuel supply interlocks also have been proposed for use with park brake systems to disable the fuel supply if the park brake is applied. For example, U.S. Pat. No. 4,295,540 relates to a vehicle brake and engine interlock that suppresses power output at the engine if the park brake is engaged, such as by adjusting the throttle valve of a carburetor to stall or shut down the engine. Other interlocks have been designed to prevent engagement or maintain or establish a neutral condition of a vehicle transmission while the park brake is engaged. For example, U.S. Pat. No. 4,051,915 relates to a neutral start and park brake safety tractor park brake force sensing system that stops current to the ignition coil if the transmission is engaged in forward or reverse while the park brake is also engaged. U.S. Pat. No. 6,886,677 relates to a mechanism that locks the forward and reverse pedals in a neutral position while the park brake is engaged. U.S. Pat. No. 6,279,937 relates to a neutral return mechanism that will place the transmission in neutral while the park brake is applied.
These interlock device, however, do not warn the operator if the park brake is adjusted incorrectly and cannot hold the vehicle in place. A tractor park brake force sensing system is needed that provides an indicator to the operator if the park brake can hold the vehicle in place, and warns the operator if the park brake is adjusted incorrectly and cannot hold the vehicle in place, or if the park brake is set or not released while the tractor is driven.